1. Field of the Invention
The present invention relates to an optical fiber product usable for a WDM (Wavelength Division Multiplexing) transmission system, having a structure for effectively restraining a fabrication error in a fabrication step and an installation step of the optical cable, and further relates to a method of evaluating a characteristic of the inventive optical fiber product.
2. Description of the Related Art
In a WDM transmission system, high-speed and large-capacity optical communication is enabled by using WDM signal light having multiplexed two or more signal channels of wavelengths different from each other. It is desired that an optical transmission line laid in each repeating section of the WDM transmission system has a superior signal transmission characteristic in a signal wavelength region (for example, in a wavelength band of 1.55 μm). Therefore, an optical fiber whose transmission characteristic changes along the longitudinal direction is proposed as an optical transmission line having a superior signal transmission characteristic.
For example, an optical fiber transmission line disclosed in U.S. Pat. No. 5,894,537 (Document 1) discloses is constituted by a unitary DMF (Dispersion-Managed Fiber) having a plurality of positive dispersion portions and a plurality of negative dispersion portions alternatively arranged so as to be adjacent to each other along its longitudinal direction. Each of the positive dispersion portions has a positive chromatic dispersion, and each of the negative dispersion portions has a negative chromatic dispersion in the signal wavelength region. When the absolute value of the chromatic dispersion in each of the positive and negative dispersion portions is set larger, a nonlinear optical phenomenon, four-wave mixing in particular, is suppressed. When the absolute value of average chromatic dispersion on the whole optical fiber transmission line is set smaller, the deterioration of transmission quality caused by the accumulated chromatic dispersion is also suppressed. The optical fiber transmission line disclosed in Document 1 is required to have a small average chromatic dispersion on the whole transmission line. Therefore, it is necessary to correctly set the length ratio the positive dispersion portion and the negative dispersion portion to a predetermined value, respectively.
However, even when the optical fiber transmission line is designed and fabricated so as to obtain a desired average transmission characteristic on the whole transmission line, there are some cases such that the average transmission characteristic on the whole optical fiber transmission line can not become a desired value after cut off, at the time that end parts of the optical fiber transmission line are cut off in a fabrication step of an optical cable and installation step of the optical cable. For example, in the fabrication step of the optical cable using the optical fiber transmission line as described above, both end parts of each optical fiber are cut off respectively until a desired condition is obtained in each step of assembling optical fiber transmission lines, water pressure resistant copper pipe welding, and sheath extrusion operation. The fiber lengths (referred to as “cut lengths” hereafter) of the both end parts cut off in each step are 100 m or more in total. Before and after cutting, the average transmission characteristic on the whole transmission line changes in accordance with the cut lengths. Particularly, in the transmission line having large absolute values of chromatic dispersion generated locally, the change of the average transmission characteristic (average chromatic dispersion) of the whole optical fiber transmission line is large before and after cutting.
U.S. Pat. No. 6,421,484B1 (Document 2) discloses an optical fiber transmission line having surplus portions connected to both ends of the use portion (portion applied to the optical fiber transmission line) beforehand. The surplus portions have a chromatic dispersion of substantially 0 ps/nm/km with respect to a wavelength in use and serve as cut portions for each work in the fabrication step of the optical cable.
Specifically, as shown in FIG. 1A, the optical fiber transmission line 2 described in Document 2 as described above, includes an effective use portion 200 and surplus portions for working 210a and 220a connected to both ends of the effective use portion 200. The effective use portion 200 includes a first region 210 and a second region 220 which are fusion-spliced at a fusion-splicing point 230. The first region 210 has a positive chromatic dispersion and the second region 220 has a negative chromatic dispersion with respect to a predetermined wavelength within a signal wavelength band (for example, wavelengths of 1550 nm).
An optical fiber fabricator delivers the optical fiber transmission line 2 having the above-described structure to an optical cable fabricator, while the optical fiber is wound around a take-up bobbin 240 as shown in FIG. 1B.